Wednesday, January 9, 2013

Overweight and Low Mortality

Last week the Journal of the American Medical Association (JAMA) published a meta-analysis documenting that being “overweight was associated with significantly lower all-cause mortality.” (here).  The study was authored by Dr. Katherine Flegal of the U.S. Centers for Disease Control and Prevention (CDC) and colleagues at the National Cancer Institute and the University of Ottawa. 
This post will focus on the study’s scientific findings and the reaction to same, which is not dissimilar to the research and backlash surrounding tobacco harm reduction.

Dr. Flegal and colleagues conducted an extensive search for population studies reporting all-cause mortality according to categories of the body mass index (BMI), which is weight in kilograms divided by height in meters squared.  They used the following standard categories: 

Table 1. BMI Ranges for Weight Categories in the Flegal Meta-Analysis
BMI RangeWeight Category
Under 18.5Underweight
18.5 to less than 25Normal
25 to less than 30Overweight
30 to less than 35Grade 1 Obesity
35 to less than 40Grade 2 Obesity
40+Grade 3 Obesity
Dr. Flegal’s literature search initially identified 7,036 articles, of which 97 met the criteria for their study, which included adequate adjustment for age, gender and smoking.  The results were reported as hazard ratios (HRs), which are similar to relative risks.

Table 2. Hazard Ratios (HRs) for All-Cause Mortality According to Weight Categories in the Flegal Meta-Analysis
Weight CategoryHR (95% Confidence Interval)
UnderweightNot Reported
Normal WeightReferent Group
Overweight0.94 (0.91 – 0.96)
Grade 1 Obesity0.95 (0.88 – 1.01)
Grade 2-3 Obesity1.29 (1.18 – 1.41)
All Obesity1.18 (1.12 – 1.25)
The major finding is remarkable: Overweight people had a LOWER risk of mortality than people of normal weight.  In addition, Grade 1 obesity (up to BMI < 35) was also associated with lower mortality risk, although this was not statistically significant.  

The results run counter to the dominant public health message that overweight and obesity are killers, which is why the study was immediately denounced by many.  As Christopher Snowdon pointed out (here), extensive media coverage of the attacks gave credence to the notion that the analysis was flawed and unreliable.  Walter Willett, professor of public health at Harvard, declared: “This study is really a pile of rubbish and no one should waste their time reading it.” (here). 

This high-profile assault is similar to that which has been waged against tobacco harm reduction findings over a 20-year period.  In the 1990’s, Philip Cole and I published numerous scientific articles on the gaping difference in health risks associated with smokeless tobacco use and smoking.  We were sometimes ignored and often reviled; ad hominem attacks routinely obscured our important findings.
I have long had a professional interest in population studies of weight and health. In 2004, I published the first and only study showing that Swedish men who quit smoking by switching to snus avoided the weight gain usually seen with smoking cessation (abstract here).  My research experience provides some insights into the Flegal/CDC study.

Why are overweight people at less risk of dying than those of normal weight?  It’s possible that the normal-weight group included people who had lost weight because they were ill and were close to death, thereby raising the death rate in the reference group to which all other categories were compared.  In other words, an artificially higher death rate in the normal-weight group resulted in an artificially lower rate among overweights.  Although this is one of the more common criticisms discussed in the media, it is not likely to be a major factor, because the vast majority of the normal-weight population are healthy.

A more likely explanation for Dr. Flegal’s surprising result is that many in her “overweight” population belong in a properly defined “normal” weight category.

In my weight research I had to establish BMI categories.  A literature review revealed that major health organizations had changed the definition of overweight.  Up to the late 1990s, researchers had defined overweight as a BMI of 27+, which meant that normal weight went from 18.5 up to 27.  Dr. Flegal agreed with this definition, as she published a study using this classification in 1994 (here).
For reasons that I was never able to pin down, the definition of overweight changed in the late 1990s to a BMI of 25+.  Suddenly, people with a BMI from 25 to 27, who were previously normal weight, were now overweight.  The prevalence of overweight skyrocketed.

Since the newly-classified overweights continued to die at normal-weight rates, this reclassification might help explain Dr. Flegal’s finding of low mortality in the overweight category. 
One unresolved problem with Dr. Flegal’s analysis is that she defined three distinct obesity categories (1, 2 and 3 in Table 1), but then combined the mortality results for the highest two categories, and failed to report any results for the underweight category.  These are important omissions which Dr. Flegal needs to explain.
Despite these issues, CDC scientists should be commended for publishing this profoundly important finding: Compared with normal-weight individuals, overweight and slightly obese people have LOWER mortality.  It’s a shame that their colleagues at the CDC Office of Smoking and Health aren’t equally forthcoming about the minuscule risks for smokeless tobacco use.  For example, the first entry on the smokeless tobacco fact sheet from the CDC (available here) is - inexplicably – a webpage for gutka, a southeast Asian tobacco product that is rarely used in the U.S.  The rest of the CDC document conflates information regarding the health risks from smokeless tobacco use with those from smoking.

Like tobacco use, overweight and obesity are the objects of revulsion among some health advocates.  Scientific evidence, however, clearly shows that overweight or minimally obese people have lower mortality risks than those of normal weight, placing them at risk only for misguided, unscientific public health interventions like fat taxes (here) or soda restrictions (here).  Like tobacco users, the overweight and obese deserve health policies based on scientific facts, not fanaticism. 


Vocal EK said...

I have to wonder whether pharmaceutical companies had any influence in lowering the standards for "overweight" and "obesity." The same thing has happened with the "normal" designations for blood glucose and total cholesterol. Since the drug companies have pills that affect these measures, it's more profits for them is the number of people designated as "abnormal" grows larger.

My mother had a BMI of 30 or 31 and she lived to be 88 years old.

Samuel Dennis said...

BMI measurements do not account for muscle mass, lean body mass, fat mass or hour-to-hour fluctuations in body weight.
This study [finding an inverse association between obesity and mortality] is scientifically worthless.